Directx 12

Funny thing is, the only games that are DX9 and DX11 only, without supporting 10 at all, are games that artificially lock out DX10 support.

That would suggest that DirectX 11 has no proprietary, DX11-only calls/functions that would otherwise prevent DX11 games/modes from functioning on a DX10 framework/system. It would also suggest that developers feel that making graphics options available to Windows Vista users was a cost-effective choice.